1. Field of the Invention
The present invention relates to an apparatus and a method for increasing semiconductor device density. In particular, the present invention relates to a method for producing vertically superimposed multi-chip devices usable with combined flip chip, wire bond, and/or tape automated bonding ("TAB") assembly techniques to achieve densely packaged semiconductor devices.
2. State of the Art
Definitions: The following terms and acronyms will be used throughout the application and are defined as follows:
BGA - Ball Grid Array: An array of minute solder balls disposed on an attachment surface of a semiconductor die wherein the solder bals are refluxed for simultaneous attachment and electrical communication of the semiconductor die to a printed circuit board. A BGA may also employ conductive polymer balls.
COB - Chip On Board: The techniques used to attach semiconductor dice to a printed circuit board, including flip chip attachment, wirebonding, and TAB.
Flip Chip: A chip or die that has a pattern or array of terminations spaced around the active surface of the die for face down mounting of the die to a substrate.
Flip Chip Attachment: A method of attaching a semiconductor die to a substrate in which the die is inverted so that the connecting conductor pads on the face of the device are set on mirror-image pads on the substrate (such as a printed circuit board), and bonded by solder reflux or a conductive polymer curing.
Glob Top: A glob of encapsulant material (usually epoxy or silicone or a combination thereof) surrounding a semiconductor die in a COB assembly.
PGA - Pin Grid Array: An array of small pins extending substantially perpendicular from the major plane of a semiconductor die, wherein the pins conform to a specific arrangement on a printed circuit board or other substrate for attachment thereto.
SLICC - Slightly Larger than Integrated Circuit Carrier: An array of minute solder balls disposed on an attachment surface of a semiconductor die similar to a BGA, but having a smaller solder ball pitch and diameter than a BGA.
TAB - Tape Automated Bonding. Conductive traces are formed on a dielectric film such as a polyimide (the structure also being termed a "flex circuit"), and the film is precisely placed to electrically connect die and a circuit board or leadframe through the traces. Multiple connections are simultaneously effected.
State-of-the-art COB technology generally consists of three semiconductor die-to-printed circuit board conductive attachment techniques: flip chip attachment, wirebonding, and TAB.
Flip chip attachment consists of attaching a semiconductor die, generally having a BGA, an SLICC or a PGA, usually to a printed circuit board, although flip chip attachment to leadframes is also known. With the BGA or SLICC, the solder or other conductive ball arrangement on the semiconductor die must be a mirror-image of the connecting bond pads on the printed circuit board such that precise connection is made. The semiconductor die is bonded to the printed circuit board, such as by refluxing the solder balls or curing the conductive polymer. With the PGA, the pin arrangement of the semiconductor die must be a mirror-image of the pin recesses on the printed circuit board. After insertion, the semiconductor die is generally bonded by soldering the pins into place. An under-fill encapsulant is generally disposed between the semiconductor die and the printed circuit board for environmental protection and to enhance the attachment of the die to the board. A variation of the pin-in-recess PGA is a J-lead PGA, wherein the loops of the J's are soldered to pads on the surface of the circuit board.
Wirebonding and TAB attachment generally begins with attaching a semiconductor die, usually by its back side, to the surface of a printed circuit board with an appropriate adhesive, such as an epoxy. In wirebonding, a plurality of bond wires are attached, one at a time, to each bond pad on the semiconductor die and extend to a corresponding lead or trace end on the printed circuit board. The bond wires are generally attached through one of three industry-standard wirebonding techniques: ultrasonic bonding--using a combination of pressure and ultrasonic vibration bursts to form a metallurgical cold weld; thermocompression bonding--using a combination of pressure and elevated temperature to form a weld; and thermosonic bonding--using a combination of pressure, elevated temperature, and ultrasonic vibration bursts. The die may be oriented either face up or face down (with its active surface and bond pads either up or down with respect to the circuit board) for wire bonding, although face up orientation is more common). With TAB, ends of metal leads carried on an insulating tape such as a polyimide are attached to the bond pads on the semiconductor die and to corresponding lead or trace ends on the printed circuit board. An encapsulant is generally used to cover the bond wires and metal tape leads to prevent contamination; TAB assemblies may be similarly encapsulated.
Higher performance, lower cost, increased miniaturization of components, and greater packaging density of integrated circuits are ongoing goals of the computer industry. Greater integrated circuit density is primarily limited by the space or "real estate" available for mounting dice on a substrate such as a printed circuit board. Conventional leadframe design inherently limits package density for a given die size because the die-attach paddle of the leadframe must be larger than the die to which it is bonded. The larger the die, the less space that remains around the periphery of the die-bonding pad for wire bonding. Furthermore, the wire bonding pads on the standard leadframe provide anchorage for the leads when the leads and the die are encapsulated in plastic. Therefore, as the die size is increased in relation to a given package size, there is a corresponding reduction in the lateral depth along the sides of the package for the encapsulating plastic which joins the top and bottom of the plastic body at the mold part line and anchors the leads. Thus, as the leads and encapsulant are subjected to the normal stresses of subsequent trimming, forming and assembly operations, the encapsulating plastic may crack, compromising package integrity and substantially increasing the probability of premature device failure.
A so-called "leads over chip" (LOC) arrangement eliminates the die-attach paddle of the leadframe and supports the die by its active surface from the inner lead ends of the leadframe. This permits a wider variety of bond pad patterns on the die, extends the leads-to-encapsulant bond area and, with appropriate design parameters, can reduce the size of the packaged device for a given die size.
One method of increasing integrated circuit density is to stack dice vertically. U.S. Pat. No. 5,012,323 issued Apr. 30, 1991 to Farnworth teaches combining a pair of dice mounted on opposing sides of a leadframe. An upper, smaller die is backbonded to the upper surface of the leads of the leadframe via a first adhesively coated, insulated film layer. A lower, larger die is face-bonded to the lower leadframe die-bonding region via a second, adhesively coated, insulative film layer. The wire-bonding pads on both upper dice and lower dice are interconnected with the ends of their associated lead extensions with gold or aluminum bond wires. The lower die must be slightly larger than the upper die so that the die pads are accessible from above through a bonding window in the leadframe such that gold wire connections can be made to the lead extensions. This arrangement has a major disadvantage from a production standpoint, since the different size dice require that different equipment produce the different dice or that the same equipment be switched over in different production runs to produce the different dice.
U.S. Pat. No. 5,229,647 issued Jul. 20, 1993 to Gnadinger teaches stacking wafers and using non-mechanically bonded electrical connections effected by metal-filled through holes contacting aligned conductive bumps of an adjacent wafer.
U.S. Pat. No. 5,291,061 issued Mar. 1, 1994 to Ball teaches a multiple stacked die device containing up to four stacked dice supported on a die-attach paddle of a leadframe, the assembly not exceeding the height of current single die packages, and wherein the bond pads of each die are wirebonded to lead fingers. The low profile of the device is achieved by close-tolerance stacking which is made possible by a low-loop-profile wirebonding operation and thin adhesive layers between the stacked dice.
U.S. Pat. No. 5,323,060 issued Jun. 21, 1994 to Fogal et al. teaches a multichip module that contains stacked die devices, the terminals or bond pads of which are wirebonded to a substrate or to adjacent die devices.
U.S. Pat. No. 5,422,435 to Takiar et al. teaches stacked dice having wire bonds extending to each other and to the leads of a carrier member such as a leadframe.
U.S. Pat. No. 5,399,898 issued May 21, 1995 to Rostoker teaches multichip, multitier semiconductor arrangements based on single and double-sided flip-chips. Using these dice to form a stacked dice package eliminates the need for wirebonding and thus reduces the size of the stacked dice package. However, these dice stacks require double-sided flip-chips which are expensive and difficult to manufacture.
See also U.S. Pat. Nos. 5,146,308; 5,252,857; and 5,266,833 for additional background regarding die configurations and assemblies employing dice.
Therefore, it would be advantageous to develop a technique and assembly for increasing integrated circuit density using non-customized die configurations in combination with commercially-available, widely-practiced semiconductor device fabrication techniques.